1. Field of Invention
The present invention relates to wireless communications and is particularly applicable, but not limited to, devices and modules for correcting errors introduced to a wireless signal after its transmission.
2. Description of the Related Prior Art
The ongoing revolution in communications technology has led to advances in wireless or radio technology. Wireless technology is currently being used in such disparate fields as wireless computer networks and wireless telephony. One technology which has been favoured for these applications is called Orthogonal Frequency Division Multiplexing (OFDM). As will be appreciated by those in the art, Frequency Division Multiplexing (FDM) involves using different frequencies to carry different signals, but this normally also involves providing a large “guard band” between the different frequencies. With OFDM, intersymbol interference (ISI) or interference between the different signals due to the closeness of their carrier frequencies is reduced as a special set of signals is used to build the transmitted composite signal. As a result of this feature, smaller guard bands are required and a more efficient use of bandwidth/resources can be achieved.
While OFDM has been successful in increasing the amount of data which is now transmitted over wireless links, much like any wireless technology, it is still subject to the limitations of the medium. Since signals are transmitted by radio waves, these signals may still suffer from not only distortions introduced by the act of transmission itself, but also from possible errors introduced by signal processing. Time-varying channel distortions, residual carrier frequency offsets, and residual sampling frequency offsets can distort and corrupt each OFDM signal.
A time domain OFDM signal consists of frames of samples corresponding to the data symbols (data frames), each frame preceded by a cylic prefix of known length (a guard frame). At the input of the Fast Fourier Transform (FFT), the time domain signal is framed in accordance with the positioning of the data and guard frames in the signal.
Referring to FIG. 1, in a typical operating environment, transmitted radio waves comprise of a number of subcarrier signals are reflected off various surfaces. As a result, the signal received at a base station may come from different directions (depending on reflections) with different strength (depending on attenuations), and the receiver sees only the combinations of all these reflections. This phenomenon is called multipath. The main problem of multipath is that it creates delay spread. Depending on the number of reflections and the propagation speed in different signals, all these signals generally do not arrive exactly at the same time at the receiver. The main technique to overcome delay spread is equalization. An equalizer attempts to invert or correct the effects of transmission distortion and signal processing errors. More specifically, an equalizer is a digital circuit that attempts to estimate the distortion due to the signal propagation through different paths, which effects each component's relative timings, phase and strength. The equalizer estimates the distortion and removes the distortion. A digital equalizer is often implemented as a time-domain digital finite response filter (FIR) adaptive taps, or as a frequency domain filter with an adaptive tap for each frequency point of concern.
More specifically, under the IEEE 802.11a standard relating to low power wireless devices, a data symbol baring 64 samples is preceded by a 16 sample cyclic prefix, yielding a total of 80 samples. A 64-point FFT is applied to a frame of 64 samples (an FFT frame). To reduce inter-symbol-interference, an FFT framing offset with respect to the start of the data frame is applied. With an FFT framing offset of 5 samples, the FFT frame of length 64 would then start at the 12th sample of the guard frame and end at the 59th sample of the data frame.
An FFT offset is, in fact, a negative delay of the FFT frames with respect to the data frames. When applied during channel estimation, the offset therefore introduces a phase rotation corresponding to that delay, a rotation which is in addition to any other phase and gain distortion already present in the channel without the delay. It is the sum of these distortions which an equalizer attempts to correct. The channel distortions include distortion due to multipath, as well as distortions due to the analogue and digital signal processing in the transmitter and receiver.
The phase rotation due to the delay causes a linear distortion on the real and imaginary components of each subcarrier's channel estimate, which makes channel estimate smoothing difficult if not impossible.
Equalizers for wireless communications systems are known in the art. For example, U.S. Pat. No. 6,327,302 entitled “Systems and Methods of Digital Wireless Communication Using Equalization” issued Dec. 4, 2001 and assigned to Ericsson Inc., describes a method and apparatus to accomplish fast adaptive equalization of a wireless communication channel utilizing time varying adaptive filter coefficients and convergence parameters. Although the technique described in this patent for a time domain equalizer works adequately, it is focused solely on overcoming channel distortions and does not address possible errors introduced by residual carrier frequency offsets, and residual sampling frequency offsets.
Another prior art equalizer is described in U.S. Pat. No. 6,389,062 entitled Adaptive Frequency Domain Equalizer Circuits, Systems and Methods For Discrete Multitone Based Digital Subscriber Line” issued May 14, 2002 and assigned to Texas Instruments Inc. This patent illustrates a channel equalization system using a combined frequency domain equalizer for use in a digital subscriber line (DSL) modem. The system adjusts for amplitude and phase distortions in a received signal but does not does not address possible errors introduced by residual carrier frequency offsets, and residual sampling frequency offsets. It is also not adapted for a wireless network utilizing OFDM.
Based on the above, there is therefore a need for a device which overcomes channel distortions and compensates for possible errors introduced by residual carrier frequency offsets and residual sampling frequency offsets. Ideally, such a device can compensate for all three deleterious effects, while being self-contained and monolithic.